Calculating the percentage change of your investment is quite easy. All it takes is a little bookkeeping and either a simple calculator or a pad of paper for doing the long division. Here is what you need to do:
Take amount that you have gained on the investment and divide it by the amount invested. To calculate the gain, take the price for which you sold the investment and subtract from it the price that you initially paid for it. Now that you have your gain, divide the gain by the original amount of the investment. Finally, multiply your answer by 100 to get the percentage change in your investment.
If the percentage is negative, resulting from the market value being lower than the book value, you have lost on your investment. If the percentage is positive, resulting from market value being greater than book value, you have gained on your investment.
Here is what the formula looks like:
(Price Sold  Purchase Price) Ã· (Purchase Price)
For example, if you bought 100 shares of Intel Corp. (INTC) @ $30/share on May 18, 2016, your total investment value is $3,000. If you sold the 100 shares on May 17, 2017, for $38/share, your proceeds from the sale would be $3800. Your realized gain can be calculated as ($3800  $3000) / $3000 = 26.67%. Alternatively, you can calculate your gain using the per share price, which would look like ($38  $30) / $30 = 26.67%.
If you just want to know how much loss or gain you've made so far without selling, the same process works and you can use the current market price in the place of the price sold, but the gain (or loss) calculated would be an unrealized gain (or loss).
This basic formula is used every day to find out exactly how many percentage points indexes, stocks, interest rates, and so on have changed over a given period of time. For example, if the Dow Jones Industrial Average (DJIA) opens at 9,000 and closes at 9,300 today, the formula would show that the percentage change over the day was 3.33% [(9,300 â€“ 9,000) / (9,000)].
However, investing does not come without costs and this should be reflected in the calculation of your percent gain or loss. The above is an illustration of the calculation without costs, such as commissions and taxes. To incorporate costs, reduce the gain (market price  price purchased) by the costs of investing. By incorporating these costs you will get a more accurate representation of your gain or loss.
Also, if your investment paid out any income or distributions, such as a dividend, you will need to add this amount to the gain amount.
Here is a more detailed way to calculate gain or loss:
[(Amount Sold  Amount Paid) + Income Gain  Costs] Ã· Amount Paid
Learn how to figure out your cost basis on an investment by reading How do I figure out my cost basis on a stock investment?

How do I calculate the percentage gain or loss for my portfolio when all of the stocks ...
Finding the total percentage gain or loss on a portfolio requires a few simple calculations. First, you should understand ... Read Answer >>

Investing
Calculating The Gain Or Loss On An Investment
Calculating the percentage of change in an investment is easy. Take the amount the investment gains and divide it by the amount invested. 
Financial Advisor
Top Tips for Deducting Stock Losses
Investors who know the rules can turn their losing picks into tax savings. Here's how to deduct your stock losses. 
Taxes
Tax Tips For the Individual Investor
Keep more of your money in your pocket with these seven guidelines. 
Taxes
Do You Have to Pay Taxes on Home Sales?
Taxes are only paid on home sales if your proceeds exceed a certain amount. 
Taxes
Capital Gains Tax 101
Find out how taxes are applied to your investment returns and how you can reduce your tax burden. 
Taxes
Here's How to Deduct Your Stock Losses From Your Tax Bill
Learn the proper procedure for deducting stock investing losses, and get some tips on how to strategically take losses to lower your income tax bill. 
Investing
Calculating The Means
Learn more about the different ways you can calculate your portfolio's average return. 
Investing
4 Behaviors That Sabotage Your Investment Goals
If you can't figure out why you're not achieving the returns you want, these behaviors might explain why. 
Managing Wealth
Tailoring Your Investment Plan
Start your own investing adventure with the help of some simple guidelines. 
Investing
Portfolio Management For the Under30 Crowd
Young investors have some advantages over their older counterparts. Read on to learn how to build a portfolio that will grow with you.

Purchase Price
The price that an investor pays for a security. This price is ... 
Paper Profit (Paper Loss)
Unrealized capital gain (or capital loss) in an investment. It ... 
Recognized Gain
When an investment or asset is sold for an amount that is greater ... 
Capital Gain
1. An increase in the value of a capital asset (investment or ... 
Capital Loss
The loss incurred when a capital asset (investment or real estate) ... 
Return
The gain or loss of a security in a particular period. The return ...